The main reason Congress initially refused to annex Texas, as supported by the information on the map, is B. if the United States annexed Texas, slave states would almost certainly outnumber free states.
The map showing free and slave states and territories in 1845 indicates that annexing Texas, a slave state, would increase the political power of slave states in Congress, thereby potentially jeopardizing the balance between free and slave states, which was a significant concern at that time.